■ Ml Mill Mill Mill Mill II lllll 111 lllll llll Ml Mil II ^ 

020 187 557 2 • 



Hollinger Corp. 
pH 8.5 



Proportioning 

Concrete Mixtures 

and 

Mixing and Placing 
Concrete 



Published by 

Portland Cement Association 

111 West Washington Street 
Chicago 

April, 1916 

"Concrete for Permanence" 



T/l 6S/ 



Proportioning Concrete Mixtures 

and 

Mixing and Placing Concrete 

What Concrete Is 

Concrete is an artificial stone prepared by mixing Portland cement, 
sand, and pebbles or broken stone in certain proportions, depending upon 
the nature of the work for which the concrete is to be used. When this 
mixture is allowed to remain undisturbed, it will become as hard as stone 
and will assume the shape of the form or mold in which placed. 

Almost every one has more or less knowledge of the possible uses of 
concrete, yet a great many persons who know of its numerous applications 
in building construction, fail to appreciate the fact that success in the use 
of concrete depends very largely upon the qualities of the sand and pebbles 
or broken stone of which the concrete is largely composed. 

Factors Governing the Quality of Concrete 

Many people believe that the quality of concrete depends entirely upon 
the Portland cement. While good Portland cement is necessary if good 
concrete is to result, nevertheless, Portland cement as manufactured today 
is a very reliable product. It is manufactured to meet exacting specifica- 
tions that have been made as the result of very extensive engineering studies 
and experiments. When dissatisfaction results from concrete work — as 
happens occasionally — the fault can almost invariably be traced, not to the 
cement, but to failure to observe some of the well-established rules or 
methods for the selection of sand and pebbles or broken stone, neglect 
of proper proportioning of materials, too much or too little water used for 
mixing, dirty water, too little mixing of materials, or lack of care used to 
place the concrete properly and protect it while hardening. 

How Portland Cement Is Packed 

No cement should be used in concrete construction except Portland 
cement of standard manufacture, such as is produced by all of the well- 
known cement manufacturers. All mills now pack Portland cement in 
standard packages (cloth sacks and paper bags) weighing 94 pounds net 
and considered as 1 cubic foot when proportioning mixtures by volume, 
which is the common method. Four of such sacks or bags make a barrel. 
Cloth sacks are billed to the cement purchaser at 10 cents each, or 40 cents 
to the barrel. When emptied, they may be returned to the dealer from 
whom the cement was purchased or to the mill, and the manufacturer will 
buy them back at 10 cents each if they are in good condition and suitable 
for further use as cement containers. A cement sack which has been wet, 
torn or otherwise rendered unfit for use, will not be redeemed. Although 
cement is sometimes packed in paper bags, this practice is growing less 
common. A charge of 10 cents per barrel (2J^ cents per bag) is made for 
packing cement in paper bags. These, of course, are not redeemable. 

Storing Portland Cement 

Portland cement should be kept in a dry place until used. It should 
never be piled directly on the ground nor in any place where it will be subject 
to the effect of moisture. A dry, tight shed having a floor raised from the 
ground so that dampness will be avoided and good circulation of air secured 



/ Pifrfi 



PROPORTIONING CONCRETE MIXTURES 



beneath the floor, are requirements of storage accommodations. It is also 
well to avoid piling sacked cement directly against the side of the shed, 
as dampness may pass through the side walls. A properly built concrete 
storage shed or warehouse, will be air and moisture-proof, and is best for 
cement storage. Cement which has become caked or hardened as a result 
of absorbing moisture must not be used in concrete mixtures. If the lumps 
may be crushed readily under slight pressure between one's fingers, the 
cement has not been damaged. Such lumpiness is usually the result of 
what is called "storage caking" and is common in the lower sacks of piles 
owing to the weight of the sacks above. 




Most gravel banks present an appearance like shown in this view. The fine material (sand) and coarse 
material (pebbles) lie in layers, usually with the sand greatly in excess of the pebbles. 

Aggregates 

Many persons disregard the importance of carefully selecting the sand, 
pebbles, and broken stone, or other materials which are sometimes used 
in their place, when preparing concrete mixtures. Sand and pebbles or 
broken stone, when used in concrete, are referred to as "aggregates." Sand 
is known as "fine aggregate," while the pebbles and broken stone are called 
"coarse aggregate." Sometimes clean stone screenings below a certain 
maximum size, that is, ranging from 34 inch downward and free from dust, 
are used as fine aggregate instead of sand. 

In order to distinguish between sand and pebbles, that is, between the 
fine and the coarse aggregate, it is necessary to fix a maximum size for the 
material that is to be called sand and a minimum size for the pebbles or 
broken stone. In concrete work, sand is the material that ranges in size 
downward from 34 inch in greatest dimension, while pebbles or broken 
stone range in size from 34 inch upward. 



MIXING AND PLACING CONCRETE 




Many gravel banks are not properly stripped of overlying soil, hence loam and vegetable matter become 

mixed with the bank material. 




This pit is largely fine material, in addition to which vegetable matter has drifted down across the face 
of the pit, thus mixing objectionable foreign material with the sand. 



PROPORTIONING CONCRETE MIXTURES 



Both sand and pebbles or broken stone must be clean and well graded 
from fine to coarse, with the coarser particles predominating. They 
must also be hard and tough. Sand and pebbles may be hard, yet brittle — 
and in the latter case would not correspond to the idea of toughness, j 

Bank-Run Material 

Bank-run gravel, that is, the natural mixture of sand and pebbles 
(usually containing more or less foreign material also) as it comes from the 
ordinary gravel bank, is not suitable for concrete mixtures. No doubt 
some concrete failures have resulted from the practice of using bank-run 
gravel without screening it so that well-graded sand and pebbles could be 
mixed in definite proportions. 

Most natural deposits of so-called gravel contain nearly twice as much 
sand as is desirable in a concrete mixture, and to produce strength approach- 
ing that resulting from a concrete made by using properly proportioned 
sand and pebbles, much more cement would be necessary with the un- 
screened bank-run material. Therefore, true economy results when the 
bank-run material is screened and the sand and pebbles correctly propor- 
tioned. 

Voids, or air spaces, contained in any given bulk or volume of pebbles 
are about 45 per cent of the mass. In order to fill these air spaces and make 
a dense concrete, the amount of sand should be approximately half the 
volume of pebbles. If unscreened bank-run material is used, a greater 
quantity of cement is required to fill the voids or air spaces. The amount 
of cement used in a 1 : 2 : 4 mixture, for instance, provided the 2 cubic 
feet of sand and 4 cubic feet of pebbles are well graded is 1 sack (1 cubic 
foot), and this is sufficient, when thoroughly mixed with the 2 cubic feet 
of sand and 4 cubic feet of pebbles, to coat every particle of sand and form 
a sand-cement mortar which will fill all of the air spaces in the pebbles. 
If, however, these proportions were changed and twice as much sand as 
gravel used, the 1 sack of Portland cement would be far less than necessary 
to thoroughly coat the increased number of sand grains and fill the air 
spaces between them. So weak and porous concrete would result. 

Watertight Concrete 

Watertight concrete may be secured for all practical purposes even where 
there is a considerable "head" or pressure of water, by using well-graded, 
correctly-proportioned materials, with enough sand and cement (sand- 
cement mortar) to overfill the air spaces or voids between the pebbles. No 
two loads of bank-run material from the same pit contain the same relative 
proportions of sand and pebbles, consequently, concrete made from such 
material is not uniform, dense nor watertight, nor so strong as it should be. 

Screening Bank Run Material 

Bank-run material must be screened by separating it into at least two 
volumes, the sand being that material which will pass through a screen 
having four meshes to the linear inch (16 meshes to the square inch), the 



MIXING AND PLACING CONCRETE 7 

pebbles or the material usually referred to as gravel, being that which will 
not pass a screen having 34 -inch meshes and consisting of particles ranging 
in size from J^-inch up to as large as proper to use in the class of construc- 
tion to be performed. For foundation work, for example, 2 inches is often 
an allowable maximum size for pebbles or broken stone, while for concrete 
fence posts and other concrete products the maximum size may be fixed 
at % inch for fence posts, silo staves, concrete block, large concrete pipe and 
tile, and V/i or 1^ inches for other work. 




Bank-run material should always be screened before using in a concrete mixture. 

A common rule in concrete construction is that no pebble or particle 
of broken stone used in a concrete mixture shall exceed in greatest dimen- 
sion, one-half the thickness of the concrete section in which it is to be used. 
This refers more particularly to concrete products such as pipe and tile, 
but does not mean, however, that such a maximum size may always be used. 



Clay or Other Foreign Material in Aggregates 

Many conflicting and sometimes confusing opinions are advanced as 
to the effect produced on concrete by the presence of a small amount of 
clay, loam or other foreign material. Certain principles, however, are 
fairly well agreed upon. When clay exists as a coating on the particles of 
sand or pebbles, it is undoubtedly injurious, as it will prevent a proper 
adhesion or bond between the cement and the sand or pebbles. For this 
reason, one should be careful to use only clean sand and pebbles. Rotted 
vegetable matter, usually spoken of either as loam or organic matter, is 
also injurious. A very small quantity of such material tends to prevent 
the cement from performing its bonding or binding action, by keeping 



8 



PROPORTIONING CONCRETE MIXTURES 



the particles of cement apart. Usually, concrete made with sand containing 
loam hardens very slowly and sometimes will not harden enough to permit 
the concrete being put to its intended use. Washing and screening bank- 
run material is neither a difficult nor an expensive operation and can easily 
be accomplished wherever there is running water. Any means which will 
agitate the materials and allow the lighter foreign material to run off with 
the water, will accomplish the purpose. 



, 8aff/i -i>t?arcr 




A simple washing trough with screen at the lower end, by means of which dirty bank-run material can 
easily be washed free from clay or other foreign material and the sand separated from the pebbles. The 
platform on which the sand and pebbles are discharged should be sloped slightly to cause the wash water 

to flow away freely. 



Washing Aggregates 

An accompanying illustration shows a washing and screening device 
which has been successfully used to remove foreign matter from sand and 
bank-run gravel before using them in a concrete mixture, and at the same 
time to separate the sand and pebbles from each other. The materials to 
be washed are shoveled into the trough at the high end, while water is 
applied through a hose connected to the pipe shown, which causes the 
materials to be tumbled and rolled about until they reach the lower end 
of the trough where the screen separates the sand from the pebbles, and 
the water carries off the foreign material. Wedge-shaped cleats (riffles) 
are nailed on the bottom of the trough inside to assist in tumbling the 
materials as they roll down. There are, of course, more elaborate means 
of washing sand and pebbles; but most of these involve power-operated 
screens and other machinery, of interest principally to the contractor or 
commercial sand and gravel producer. 



MIXING AND PLACING CONCRETE 



Miscellaneous Aggregates 

So far only sand and pebbles or broken stone have been mentioned as 
possible concrete aggregates. Usually the home worker finds sand and 
pebbles the materials easiest to obtain. Yet there are many sections of the 
country where suitable pebbles are not obtainable, and the farms or fields 
are littered with "rubblestones," or "nigger heads" which are a great hin- 
drance to cultivating land that would naturally be productive if these stones 
were out of the way. When properly crushed, these will often make a 
suitable grade of broken stone that can be used as a substitute for pebbles 
in constructing permanent concrete farm buildings and other structures. 
Small stone crushers operated by three or four horse-power gasoline engines 
can now be obtained at relatively low cost, and in this way otherwise 
useless material may be converted into useful material for concrete, at a 
cost ranging from 40 to 60 cents per cubic yard. Sometimes the fine 
screenings from these crushed rubblestones make a better fine aggregate 
than the only sand that may be available locally. Mine tailings, such as 
"chats" from zinc smelters, decomposed granite, such as found in sections 
of Colorado and Wyoming, and similar materials, such as tufa rock, can 
often be made available for use as concrete aggregate. 

Selecting Materials for Fire Resistance 

One of concrete's greatest merits is its fire resistance. If this is to be 
attained to the fullest possible degree, care must be given to selecting the 
sand and pebbles or broken stone to be used in the concrete mixture. Trap 
rock, some grades of pebbles, and bank slag are generally to be preferred 
if .the concrete is to possess maximum fire resistance. Sand for this 
purpose should be of what is known as a silicious nature. 

Mixing Water 

Water used in concrete mixtures should be clean. Not only should 
there be no loam, clay or other foreign material in the water, but it should 
be free from oil and excessive alkali. Water that is fit to drink is best for 
mixing concrete. Alkali in mixing water may not only be the cause of a 
whitish deposit on the finished concrete, called efflorescence, but if in 
excess, may finally affect the strength of the concrete. 

Proportioning Mixtures 

Strength and density of concrete are dependent upon correct propor- 
tioning of all materials. Strength, of course, depends in part on using 
clean, hard, durable sand and pebbles, while density depends largely upon 
correct proportioning — yet both strength and density are results of observ- 
ing the same fundamentals. 

Porportioning of materials must be such that the cement will coat every 
particle of sand, and the sand-cement mortar will coat every pebble or 
particle of broken stone and be slightly in excess of what is actually required 
to fill the air spaces or voids in the mass of pebbles. Unless these conditions 
are secured, the concrete will be porous, hence not watertight, and the 
sand and pebbles will not be firmly bound together into one mass by the 
cement. In other words, the amount of sand used should slightly exceed 
the volume of air spaces in the pebbles or broken stone, while the quantity 



10 PROPORTIONING CONCRETE MIXTURES 

of cement used should slightly exceed the voids or air spaces in the sand. 
If these principles of proportioning are carefully observed, the resulting 
concrete will be practically free from air spaces, consequently strong, 
dense and watertight. 

Compressive strength of concrete, which is the ability to carry heavy 
loads placed directly upon it, also increases with an increase in density. 

There are a number of ways to determine the volume of air spaces in a 
given bulk of sand and pebbles or broken stone, so that the materials to be 
used for any concrete mixture can be accurately proportioned to reduce 
air spaces to a minimum. Rut for extreme accuracy, rather complicated 
methods must be used, and it has therefore become a more or less general 
practice for the home worker to rely on so-called arbitrary mixtures, which 
are recommended when the sand and pebbles or broken stone have been 
selected so as to insure materials of uniform grading from fine to coarse, 
with the coarser particles predominating. 

A table of such arbitrary mixtures follows, with suggestions as to the 
particular classes of construction or parts of construction to which they 
are suited. In this table the figures stand for the volumes of the cement, 
sand, and pebbles or broken stone used. For example: A 1:2:3 mixture 
means 1 sack (1 cubic foot) of Portland cement, 2 cubic feet of sand, and 
3 cubic feet of pebbles or broken stone. 

A 1:2 mixture means 1 sack (1 cubic foot) of Portland cement and 2 
cubic feet of sand. 



TABLE OF RECOMMENDED MIXTURES 

1:1 Mixture for 

The wearing course of two-course floors subject to heavy trucking, 
such as occurs in factories, warehouses, on loading platforms, etc. 

1 :iy 2 Mixture for 

The wearing course of two-course pavements, in which case the pebbles 
or crushed stone is graded from M to 3^ inch. 

2:3 Mixture for 

Reinforced concrete roof slabs. 

One-course concrete road, street, and alley pavements. 
One-course walks and barnyard pavements. 
One-course concrete floors. 
Fence posts. 

Sills and lintels without mortar surface. 
Watering troughs and tanks. 
Reinforced concrete columns. 
Mine timbers. 

Construction subjected to water pressure, such as reservoirs, swim- 
ming pools, storage tanks, cisterns, elevator pits, vats, etc. 

2 : 4 Mixture for 

Reinforced concrete walls, floors, beams, columns and other concrete 
members designed in combination with steel reinforcing. 



MIXING AND PLACING CONCRETE 11 

Concrete for the arch ring of arch bridges and culverts ; foundations for 
large engines causing heavy loading, some impact and vibration. 
Concrete work in general subject to vibration. 
Reinforced concrete sewer pipe. 

1 : 2Y 2 :b Mixture for 

Silo walls, grain bins, coal bins, elevators and similar structures. 
Ruilding walls above foundation, when stucco finish will not be applied. 
Walls of pits or basements, subject to considerable exposure to moisture, 

but practically no direct water pressure. 
Manure pits, dipping vats, hog wallows. 
Racking of concrete block. 
Rase of two-course road, street and alley pavements. 

l:%y 2 :5 Mixture for 

Walls above ground which are to have stucco finish. 

Rase of two-course sidewalks, feeding floors, barnyard pavements and 
two-course plain concrete floors. 

Abutments and wing walls of bridges and culverts, dams, small retain- 
ing walls. 

Rasement walls and foundations for ordinary conditions where water- 
tightness is not essential. 

Foundations for small engines. 

1:3:6 Mixture for 

Mass concrete such as large gravity retaining walls, heavy foundations 
and footings. 

1 :1Y 2 Mixture for 

Inside plastering of water tanks, silos, and bin walls, where required, 
and for facing walls below ground when necessary to afford addi- 
tional protection against the entrance of moisture. 

Rack plastering of gravity retaining walls. 

1 : 2 Mixture for 

Scratch coat of exterior plaster (cement and stucco) . 
Facing block and similar concrete products. 

Wearing course of two-course walks, floors subjected only to light loads, 
barnyard pavements, etc. 

1 : 2Y 2 Mixture for 

Intermediate and finish stucco coats. 

Fence posts when coarse aggregate is not used. 

1:3 Mixture for 

Concrete block when coarse aggregate is not used. 

Concrete brick. 

Concrete drain tile and pipe when coarse aggregate is not used. 

Ornamental concrete products. 



12 



PROPORTIONING CONCRETE MIXTURES 



QUANTITIES OF MATERIALS REQUIRED FOR VARIOUS 
MIXTURES OF MORTAR AND CONCRETE 



MIXTURE MATERIALS FOR 
ONE RAG RATCH 


RESULTING 
VOLUME IN 
CURIC FEET 


QUANTITIES OF CEMENT, SAND, 
AND PERRLES OR STONE REQUIRED 
FOR ONE CURIC YARD OF COM- 
PACTED MORTAR OR CONCRETE 




Cement 
in 

Sacks 


Sand 
cu. ft. 


Pebbles 

or 

Stone 

cu. ft. 


Mortar 


Con- 
crete 


Cement 

in 

Sacks 


Sand 


Stone or Pebbles 




Cu. Ft. 


Cu. Yd. 


Cu. Ft. 


Cu. Yd. 


1:1* 




1.5 




1.75 




15.5 


23.2 


.86 






1:2 




2.0 




2.1 




12.8 


25.6 


.95 






1:2* 




2.5 




2.5 




11.0 


27.5 


1.02 






1:3 




3.0 




2.8 




9.6 


28.8 


1.07 






1:2:3 




2.0 


3.0 




3.9 


7.0 


14.0 


.52 


21.0 


.78 


1:2:4 




2.0 


4.0 




4.5 


6.0 


12.0 


.44 


24.0 


.89 


1:2*:4 




2.5 


4.0 




4.8 


5.6 


14.0 


.52 


22.4 


.83) 


l:2i:5 




2.5 


5.0 




5.4 


5.0 


12.5 


.46 


25.0 


.92 


1:3:6 




3.0 


6.0 




6.4 


4.2 


12.6 


.47 


25.2 


.94 



(Rased on tables in "Concrete, Plain and Reinforced," by Taylor & Thompson) 

MIXING CONCRETE 
Hand Mixing 

If concrete is to be mixed by hand, a watertight mixing platform, large 
enough so that two men using shovels can work upon it at one time, is 
needed. This platform should be made preferably of 2 by 6-inch plank, 
tongued and grooved so that tight joints will be formed to prevent the 
loss of cement carried away when adding mixing water to the materials. 
These planks may be nailed to three 2 by 4's set on edge. The two out- 
side ones may have holes bored near the end so that if necessary to move 







, 




mmm ■ - -SgffiM 


„ 




-^p 


Mfc '' 






^"S.-^^ST" 


1 ""■^ 


•x 








f ' 


;.;...::■. %;&> 


h 


1 


i ■ 


\~. i 


7**?-* 




*^8|j 






^^f* 


WBBBmk 


-if 


^ :: m 


. . ■ 

■•■ ■ ■ ■ ■■-.'■■ *m 

w 


^jfcsn. 



Mixing platform with bottomless box for measuring materials. This box should be marked on the inside 
to show capacities of 1, 2, and 3 cubic feet. When level full, the box holds 4 cubic feet. 



MIXING AND PLACING CONCRETE 13 



the platform from place to place, clevises and a chain may be attached so 
a horse can be hitched to the platform to drag it. Two sides and one end 
of the platform should have a strip nailed along the edge, projecting about 
2 inches above the top of the platform to prevent materials from being 
washed or shoveled off the platform while mixing. 

Whether concrete is mixed by hand or machine, a measuring box is 
necessary so that the sand and pebbles or broken stone can be properly 
proportioned. Such a device is really a bottomless box, and can be made 
of 1 or 4 cubic feet capacity; in the latter case, marks should be made at 
proper levels on the inside to indicate capacities of 1, 2 and 3 cubic feet. 
When used, the measuring box is set on the mixing platform and after the 
required amount of sand has been shoveled in and measured, the box is 
raised and the sand spread about in an even layer on the mixing platform. 
(Wheelbarrows of different capacities are also made for measuring aggre- 
gates. These are generally used when concrete is mixed by machine, 
because the materials can readily be dumped from wheelbarrows into the 
mixer hopper or loading skip. Otherwise a measuring box of 1 cubic foot 
capacity is set in an ordinary wheelbarrow and filled in the usual way.) 

Next the cement, which need not be measured, as each sack may be 
considered 1 cubic foot, is spread in an even layer on top of the sand. 
Square-pointed shovels are used to turn the cement and sand two or three 
times, or more if necessary, until the streaks of brown and gray have 
merged into a uniform color, indicating thorough mixing of the two 
materials. 

Pebbles or broken stone (first thoroughly wet) are then measured and 
spread in a layer on top of the cement and sand and all of the materials 
again mixed by turning with shovels. Then a depression or hollow is 
shoveled in the center of the pile and water added gently, preferably by 
a spray from a hose, while two or more men turn the materials with square- 
pointed shovels, adding water while this is being done, until the cement, 
sand and pebbles have been thoroughly and uniformly mixed and the 
desired consistency obtained. 

Quantity of Water Used 

If much concrete is to be mixed, it is well to determine when prepar- 
ing the first batch, the amount of water necessary to produce the required 
consistency, so that the same measured quantity of water can be used for 
subsequent batches of the same ^size, thus always producing concrete of 
the same consistency. 

For most classes of construction, the correct amount of water is that 
which will make a concrete of quaky or jelly-like consistency. Less water 
is sometimes used for foundation concrete where the wall is not to enclose 
a cellar or basement, that is, need not be watertight; but the "quaky" 
consistency is preferable wherever possible to use it. 

In hand mixing, batches should not be larger than can conveniently 
be mixed on the platform, nor larger than will represent an amount of 
concrete which can be placed within thirty minutes after mixing. 

Quaky mixtures are made to settle to all parts of the form by spading 
the concrete rather than by tamping it when placing. Spading makes 



14 PROPORTIONING CONCRETE MIXTURES 




A measured quantity of sand and cement is spread in a^ thin layer on the platform preparatory to mixing 

in the dry state. 




Men using square-pointed shovels turn the sand and cement thoroughly several times until the streaks 
of brown and gray have merged into a uniform color. 



MIXING AND PLACING CONCRETE 



15 




The previously wetted broken stone or pebbles are then added to the sand-cement mixture and thor- 
oughly incorporated with it, after which a depression or hollow is shoveled in the pile and the required 

amount of water added. 




The entire mass is then turned thoroughly several times until every pebble or particle of broken stone is 
covered with sand-cement mortar and the whole mass is of uniform consistency. 



16 PROPORTIONING CONCRETE MIXTURES 

quaky concrete settle to greatest density and releases any air that may be 
entrapped in the mixture. If concrete containing less water than would 
produce a quaky mixture is being placed, then it is tamped in the forms, 
but such concrete should be wet enough to cause water to flush to the sur- 
face under moderate tamping or compacting. 

Experience has proved that where sand is only moderately damp when 
combined with the other materials in a concrete mixture, that the quaky 
or jelly-like consistency can usually be produced by using water in the 
proportion of 1 gallon to 1 cubic foot of concrete in place. This figure is 
not intended to represent a fixed rule, but rather an approximation of the 
amount required. If the sand is dry, then it is necessary to increase the 
amount of water to obtain the desired consistency. Certain kinds of 
broken stone are somewhat porous, and if used, would take up some 
of the mixing water and necessitate more water to produce a quaky mix- 
ture. When the sand contains a large amount of moisture, naturally less 
water is required. 

Some classes of concrete work require wetter concrete mixtures than 
are described by the word "quaky." Take, for instance, concrete fence 
posts. In such work it is necessary to use a slightly wetter mixture so 
that when stirring the concrete in the molds or agitating the molds to 
compact the concrete and release entrapped air, reinforcing metal will 
everywhere be thoroughly surrounded and the concrete be enabled per- 
fectly to bond with (adhere to) the steel. In all concrete work one must be 
careful to avoid using so much water as to make the concrete sloppy or 
soupy, thus causing the sand-cement mortar and pebbles or broken stone 
to separate. 

Even though concrete mixtures may have been proportioned so that 
the combined materials contain the lowest possible volume of air spaces or 
voids and also represent a mixture rich enough for strength, yet unless the 
proper amount of water is used the resulting concrete will not be water- 
tight; therefore, the correct amount of water is very important when one 
desires to secure watertight construction — just as important, in a way, as 
having the sand and pebbles properly proportioned and well graded, and 
using the necessary quantity of cement. 

The sand and cement should not be combined very far in advance of 
adding the required amount of pebbles or broken stone, because the mois- 
ture which the sand contains will cause the cement to commence hardening 
and this will affect the final strength of the concrete. Just as soon as the 
cement and sand for a batch of concrete are mixed, the pebbles or broken 
stone and water should be added, and mixing continue without interrup- 
tion, followed immediately by placing of the concrete. 



Machine Mixing 

At the present time there are many types of power-operated concrete 
mixers on the market. These range in size from small and inexpensive 
machines, complete with self-containing gasoline engine, representing; an 
outfit such as well meets the home worker's requirements, up to the large 
self-propelling machines that are used in concrete road building. The 



MIXING AND PLACING CONCRETE 



17 



smaller machines range in price from $75 or $80 upward, and if one has 
any considerable amount of concreting to do, such machines represent a 
profitable investment and reduce the labor cost of mixing, to say nothing 
of insuring well-mixed concrete. Only a mixer of the batch type should be 
used, that is, one having a revolving drum in which all of the materials are 
placed at one time, the drum being revolved until mixing is complete. 




A small self-contained mixer-engine outfit at work on a job of mixing concrete for a concrete walk- 
an outfit is a profitable, labor-saving and efficient piece of equipment. 



Such 



Before water is added the mixer drum should be given a few revolutions 
to thoroughly mix the materials in the dry state, then the required amount 
of water should be added and the drum revolved for a specified time or a 
specified number of revolutions, either of which should represent a period 
of mixing of at least one minute. Some mixers have attached to them a 
water tank by means of which a measured quantity of water for each batch 
may be added to the dry materials. Care should be taken not to place 
more materials in the drum than recommended by the manufacturer, other- 
wise thorough mixing cannot be accomplished, nor should the mixer drum 
be revolved too rapidly, for that would cause the materials to cling to the 
inner surface of the drum rather than be tumbled about and thoroughly 
mixed. The materials for a batch should not exceed in volume one- 
third of the full capacity of the mixer drum. 

In making concrete products such as are usually formed by machines 
with iron molds, that is, concrete block, brick and tile, a somewhat dryer 
mixture must be used so that the molds can be removed immediately after 



18 



PROPORTIONING CONCRETE MIXTURES 



forming the products. In such work it is a general rule to use as much 
water as possible without interfering with the quick removal of the mold 
or having the product so wet that it will not retain its shape after the mold 
is removed. 

PLACING CONCRETE 

General 

As the hardening action resulting from the combination of cement and 
water begins very soon after a batch of concrete is mixed, concrete should 
be deposited as quickly after mixing as possible. For convenience in 
placing concrete the mixing platform (if concrete is being mixed by hand) 
or the mixer (if machine mixing is being done) should be placed near where 
the concrete is to be deposited. When the concrete is placed in a trench 
without using forms, boards or planks should be laid along and across the 
trench for the workers to stand upon when dumping and tamping concrete 
so as to prevent knocking down earth from the sides of the trench into 
the freshly placed concrete. 




If the mixing platform can be placed near where concrete is to be deposited, the concrete may be shoveled 

directly from the platform into the forms. 



Concreting work should be planned so that the quantity of concrete 
that will be placed during a working day or whatever time is to be devoted 
to concreting, can be estimated with reasonable accuracy, to enable leav- 
ing the work in suitable condition for resuming concreting later. Con- 
crete should be deposited in a layer or layers of uniform depth all around 
the enclosure, that is, between forms, unless it is desired to complete the 
work in sections to full height of forms, when a vertical joint is arranged for. 



MIXING AND PLACING CONCRETE 



19 



As already mentioned, concrete for some foundation work is mixed to a 
consistency produced by using less water than would be required for a 
quaky mixture. Such concrete is tamped in the forms. Quaky concrete 
instead of being tamped is spaded thoroughly between form faces and 
next to form faces so as to secure the greatest possible density and to 
release air that may be in the mixture. Spading next to form faces also 

forces back the coarser particles 
and allows the sand-cement mor- 
tar to come next to the forms, 
thus increasing density and 
watertightness of the surface, and 
producing a smooth finish free 
from pebble pockets. An old hoe 
straightened out, or a flattened 
garden spade with slots in it, 
makes a convenient spading tool. 
Narrower spading tools are 
needed sometimes to thoroughly 
work the concrete around rein- 
forcing rods. 



Methods of Placing 

Methods of placing concrete 
necessarily vary in accordance 
with certain working conditions 
and the nature of the construc- 
tion. Concrete for feeding floors 
and barnyard pavements, for 
example, is dumped from wheel- 
barrows upon a previously pre- 
pared foundation, and as rapidly 
as the forms for alternate slabs 
are filled, the concrete is struck 
off level by using a straightedge 
resting upon side forms and 
moved back and forth across 
them with a saw-like motion, 




An old hoe straightened out, or a flattened garden spade 

or similar tool, makes a convenient device for spading 

concrete in the forms, as shown in this illustration. 



after which the surface is finished by using a wood float. In the case of 
troughs and stock-watering tanks, necessary reinforcement ^ having been 
previously placed in the forms and secured in proper position, concrete 
for the floor or bottom of the trough or tank is placed, the inside form 
is then quickly set in position supported from the top of the outside form 
and concrete for side walls immediately placed so as to make certain that 
it will unite perfectly with that forming the floor. 



Concrete should not be placed in layers deeper than will permit com- 
pacting it firmly and causing it to unite or bond with the concrete previously 
placed. If a mixture containing less water than a quaky one is being 
placed, layers no deeper (thicker) than 6 inches should be deposited, while 



20 



PROPORTIONING CONCRETE MIXTURES 



for a quaky mixture the layers should not be deeper than the length of the 
blade of the spading tool being used. 

In constructing walls for milkhouses, smokehouses, icehouses and simi- 
lar small buildings, wall forms are filled as just described, yet for thin wall 
sections one should be careful not to board up the forms so high on one 
side as to prevent effective use of the spading tool. For instance, if the 
wall being laid is only 6 inches thick the space between forms makes it 
difficult to properly spade the concrete if both forms are boarded up to a 
height of 8 or 10 feet. Another objection to placing concrete in forms that 
have been carried up too far, is that the concrete has to drop through such 
a distance when dumped or shoveled into the forms, that there is likely to 
be some separation of the sand-cement mortar from the pebbles or broken 
stone, thus resulting in a lack of uniform density in the wall because of 
the formation of pebble pockets. 

If necessary that a certain section be finished complete to the top 
of the forms within a stated time, arrangement must be made to provide 
a vertical joint in the work by blocking a board in the forms, with a beveled 
2 by 4 or similar strip nailed to the face against which concrete is being 
placed so that this vertical jo nt will have a mortise face, as shown in the 
illustration on next page. Then when concrete is placed in the next section 
the board stop is taken out and the hardened concrete acts as an end form 
against which the fresh concrete is placed, a tenon being formed in the 
mortise of the concrete previously deposited. Before resuming concreting 
against this section, the face of the hardened concrete should be thoroughly 
painted with thick hot tar just in advance of placing the various layers of 
■concrete so that the joint will be effectively sealed against leakage. 




Sometimes wheelbarrows are used to wheel concrete from the mixing platform to where it is to be placed. 



MIXING AND PLACING CONCRETE 



21 





hfa// w/V/r for/r? 



Form 3how/ngr 
stop board /n /?/ace . 



If necessary or desirable to finish some section of concreting to the full height of the forms, a vertical joint 
may be provided for as shown in this illustration. 



Stopping Work for the Day 

When necessary to discontinue concreting before forms are filled, as at 
the close of the day, for instance, the top of the concrete last placed in the 
form should be roughened by scratching it with a stick to prepare for a good 
bond with fresh concrete that is to be placed the next day or when con- 
creting is resumed. Immediately before resuming concreting, however, 
the surface of the old concrete should be well washed off and painted with 
a mixture of cement and water, mixed to the consistency of thick cream and 
applied just in advance of placing the fresh concrete. 

Protecting the Finished Work 

Proper protection of concrete after placing is of utmost importance. 
Although a concrete mixture begins to harden within a short time after all 
of the materials have been combined, the complete changes which result 
in thorough hardening take place rather slowly and are accomplished best 
only in the presence of moisture. If concrete, after placing, is left exposed 
to sun and wind, much of the water which was mixed with it will evaporate, 
and instead of hardening properly, the concrete will simply dry out. Many 
people believe that drying out is the natural and only process concluding 
the work of concreting. The finished concrete must be so protected that 
it will retain the water already in it until complete hardening has been 
accomplished. 



22 PROPORTIONING CONCRETE MIXTURES 



Concrete floors, for example, should be covered with straw, sand, earth 
or some other protective covering and this be kept wet by sprinkling for a 
period varying from several days to two weeks or more, depending upon the 
nature of the work and weather conditions. Concrete hardens more 
rapidly in hot than in cool weather. Ruilding walls, which cannot be 
covered with straw or sand, may be protected by hanging burlap, old can- 
vas or similar material over and around them and keeping this covering 
wet. It is well also to sprinkle the concrete several times daily for the 
first few days after placing. Forms left in place provide good protection, 
but even then the work should be wet down daily. 

Care should be taken not to remove forms from concrete that is to sus- 
tain any load other than its own weight until all possibility of collapse 
shall have passed. 



LIBRARY OF CONGRESS 

020 187 557 2 



Tennis Courts of Concrete 

Concrete Septic Tanks 

Concrete Fence Posts 

Small Concrete Garages 

Concrete Feeding Floors, Barnyard 
Pavements and Concrete Walks 

Farmer's Handbook on Concrete 
Construction 

Concrete Facts About Concrete 
Roads 

Facts Every One Should Know 
About Concrete Roads 



Are new publications which may interest 
you. If you would like any of these book- 
lets, write the Portland Cement Association, 
111 West Washington Street, Chicago. 



"Concrete for Permanence" 




020 187 557 2 • 



Hollinger Corp. 
pH 8.5 



\ 



LIBRARY OF CONGRESS 



020 187 557 2 



Hollinger Corp. 
pH 8.5 



